DS224+ RAM and HDD compatibility

Hello there.

I have some experience with PCs but the NAS world is quite new to me, so I wanted to ask how literal are the RAM and HDD compatibility lists to be taken?

For example, I see that the maximum size of a single HDD varies depending on the model/manufacturer. Why is that?

So I wanted to understand what is the maximum capacity I can put on a DS224+?

Can I also install RAM from third-party manufacturers? They're definitely cheaper.

Thanks.